Ziegler-Natta (ZN) type catalysts are well known in the field of producing polymers from olefinic monomers, like ethylene (co)polymers. Generally the catalysts comprise at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC) and/or optionally an internal organic compound, like an internal electron donor compound. The ZN catalyst may also comprise further catalyst component(s), such as cocatalyst(s) and optionally external additives, like external donors.
A great variety of Ziegler-Natta catalysts have been developed to fulfill the different demands in reaction characteristics, large-scale production, and producing poly(alpha-olefin) resins of desired physical performance. One type of a typical Ziegler-Natta catalyst component is preferably comprised of a magnesium compound, an aluminium compound and a titanium compound supported on a particulate support. The particulate support can be an inorganic oxide support, such as silica, alumina, titania, silica-alumina and silica-titania, typically silica.
The catalyst component can be prepared by sequentially contacting the carrier with the above mentioned compounds, as described, for example in EP 688794 and WO 99/51646. Alternatively, it can be prepared by first preparing a solution from the components and then contacting the solution with a carrier, as described in WO 01/55230.
Another group of typical Ziegler-Natta catalysts are based on magnesium dihalide, typically MgCl2, that contain a titanium compound and optionally a Group 13 compound, for example, an aluminium compound. Such catalysts are disclosed, for instance, in EP376936, WO 2005/118655 and EP 810235.
The above described ZN-catalysts are described to be useful in olefin polymerisation, i.e. for producing ethylene (co)polymers.
However, even though many catalysts of prior art show satisfactory properties for many applications, there has been a need to modify and improve the properties and performance of the catalysts to achieve desired polymer properties and to have catalysts with desired performance in desired polymerisation processes.
Adding various molecules, such as internal organic compounds or external additives can influence the polymerization character of the catalyst and thereby the subsequent polymer properties. The internal organic compounds can be internal electron donors or other compounds having influence on the performance of the catalyst. An example of external additives is external electron donors. In the present application, the phrases external electron donor and external additive are interchangeable, and also internal electron donor and internal organic compound are interchangeable.
U.S. Pat. No. 5,055,535 discloses a method for controlling the molecular weight distribution (MWD) of polyethylene homopolymers and copolymers using a ZN catalyst comprising an electron donor selected from monoethers (e.g. tetrahydrofuran). The monoether, which is tetrahydrofuran in this case, is added to the catalytic component with the cocatalyst, at the latest, upon commencement of the polymerisation reaction, and is further characterised that under no circumstance should the monoether be brought into contact with the catalytic component without the presence of the cocatalyst in the medium.
WO2005058982 discloses a two-stage gas-phase polymerisation process for producing high density polyethylene (HDPE) in the presence of a solid Ziegler-Natta catalyst component and alkylaluminum compound as cocatalyst. Further, an external donor is added into the second gas phase reactor so that the disclosed process is then capable of producing a relatively broad molecular weight ethylene copolymer in the presence of a Ziegler-Natta catalyst capable of retaining at the same time good hydrogen sensitivity and a capability to homogeneously distribute the comonomer. Said external donor can be the same or different to the optional internal donor, and is preferably an ether, like tetrahydrofuran (THF). Alkoxysilanes are also listed among other external donors, such as alcohols, glycols, ketones, amines, amides and nitriles. The catalyst productivity is not discussed or disclosed, nor is any problem relating to the production of high Mw ethylene (co)polymer. In WO2005058982 it is only discussed the possible negative impact of external donors on the hydrogen response and consequently on the activity of the catalyst in the polymerization step, where the relatively low molecular weight the polymer is produced. However, producing high Mw ethylene (co)polymer good hydrogen response of the catalyst and/or substantial hydrogen carry-over from the reactor in which the relatively low molecular weight polymer is produced can cause problems. Moreover, it is generally known in the art that not each and every external additive improves comonomer distribution.
WO 2007051607 A1 suggests the possibility of producing a multimodal ethylene polymer by using alkyl ether type internal electron donors to modify the ZN catalyst component. The final molecular weight distribution (MWD) is narrower due to the reduction of MWD of a higher molecular weight (HMW) component. The electron donor is preferably tetrahydrofuran.
The use of alkoxysilanes as external electron donors with respect to polymerization of α-olefins, particularly, with respect to polymerization of propylene for increasing stereo-regularity/tacticity by Ziegler-Natta catalysts is commonly known in the field and is widely used in the industry, as described, for example, in U.S. Pat. Nos. 4,547,552, 4,562,173, 4,927,797, WO03106512, and EP0303704. In addition to stereo-regularity/tacticity control also other properties of the final propylene polymer may be affected by use of an external electron donor.
Alkoxysilane type external donors are not commonly used nor widely presented in patent literature in ethylene (co)polymerization. However, WO200238624 discloses that use of specific alkoxysilanes together with a haloalkane compound in ethylene polymerization in the presence of cocatalyst and a very specific solid titanium catalyst component results in PE with narrow molecular weight distribution and high bulk density with high activity. WO200238624 does not discuss polymerisation in a multistage polymerisation process or polymerisation in a gas phase reactor. All polymerisation examples describe one-step liquid-phase polymerizations.
WO2004055065 discloses a solid catalyst component comprising Ti, Mg, halogen and electron donor in specific molar ratios for the preparation of copolymers of ethylene with α-olefins, where said α-olefins are homogeneously distributed along the polymer chain. Said catalyst is used in preparing linear low density PE. The electron donor (ED) is preferably an ether, like tetrahydrofuran. The catalyst component, as defined, is used in polymerisation reactions together with an alkylaluminum compound and optionally with an external electron donor. The optional external electron donor is said to be equal to or different from the ED used in catalyst component. It can also be selected from silicon compounds of formula RaRbSi(OR)c, especially cyclohexyltrimethoxysilane, t-butyltrimethoxysilane and thexyltrimethoxysilane. The polymerization process of WO2004055065 comprises an optional pre-polymerisation step followed by a gas phase polymerization step.
CN103304869 discloses a multimodal PE composition for pipes having density of 0.935 to 0.945 g/cm3 and comprising three components 1) ethylene homopolymer (40-60 wt-%) with density more than 0.970 g/cm3 and melt flow rate 5 (MFR5) of more than 300 g/10 minutes, 2) ethylene-α-olefin copolymer (30-40 wt-%) with density of not greater than 0.935 g/cm3 and MFR5 of not greater than 1 g/10 minutes and 3) ethylene-α-olefin copolymer (5-30 wt-%) with density less than 0.935 g/cm3 and MFR5 of not greater than 0.01 g/10 minutes. Each component has narrow molecular weight distribution (Mw/Mn) of less than or equal to 5 and comonomer content of 0.2-0.7 mol-%. This composition is prepared in the presence of a Ziegler-Natta catalyst and dimethoxydiphenlysilane or cyclohexyldimethoxysilane as external donor in a multistage process comprising only slurry reactors. No information of catalyst productivity is given.
WO2013/113797 discloses similar type of multimodal PE composition as CN103304869 above having a low molecular weight ethylene polymer component and two higher molecular weight ethylene copolymer components. Polymer is produced in slurry polymerisation reactors, although other reactor types are also generally mentioned. However, no external donors are used.
WO2014102813 discloses a heterogeneous Ziegler-Natta catalyst system comprising a titanium procatalyst with a magnesium compound as a base, and at least one cocatalyst comprising at least one organoaluminium compound, a hydrocarbon medium and at least one external donor comprising at least one organosilane compound. The catalyst system is obtained by adding said organoaluminium compound and organosilane compound to the procatalyst system. The catalyst system is used for producing UHMWPE (ultrahigh molecular weight polyethylene). The polymerisation process is a one-step polymerisation.
WO2009/027270 discloses a catalyst for ethylene polymerisation comprising a solid catalyst component comprising titanium, magnesium and halogen, an aluminum alkyl cocatalyst and a silane compound. Narrow molecular weight distribution is desired indicated by FRR21/2 ratio at most 30. Use of the catalyst for producing multimodal polymer or use in a multistage process is not discussed.
Although much development work in Ziegler-Natta catalyst has been done there is still room for improvement. If specific polymer properties or specific polymerisation processes or combinations thereof are desired, catalysts of prior art do not serve as appropriate catalysts as such, but modifications and adjustments are needed in order to get polymer with desired properties and to produce said polymers with good polymerization productivity.
One method to allow the production of multimodal ethylene (co)polymers with high molecular weight fraction and broad molecular weight distribution (MWD) in a multistage process is to reduce or exclude the introduction of hydrogen as a molecular weight controlling agent to at least one of the polymerisation stages or reactors. However, if the relatively low molecular weight (co)polymer is produced in the stage before the stage, where the relatively high molecular weight copolymer is produced, it results, due to substantial hydrogen carry-over, in a relatively high hydrogen concentration in the reactor, where the relatively high molecular weight copolymer should be produced. Moreover, to provide polymer with good processability and improved flow properties, multimodal polymers with a smaller proportion of the high molecular weight fraction are often desired. However, this in turn results easily in a relatively low ethylene concentration/partial pressure and therefore higher H2/C2 molar ratios, in the reactor, where the relatively high molecular weight copolymer is to be produced. If ethylene (co)polymers with high molecular weight fraction are desired, and the amount of hydrogen has already been minimized, then external additives are added to the first polymerization stage. However, in that case, the problem is that polymers are often produced at the expense of the catalyst productivity. Further, in producing polyethylene in a multistage process comprising at least two stages one problem that is often encountered with the prior art ZN-catalysts is that it is difficult to produce an ethylene homo- or copolymer having broad molecular weight distribution (MWD) (i.e. having melt flow rate ratio FRR21/5≥40 and/or polydispersity index PDI≥27) and at the same time keep productivity at a high level. I.e. in a beneficial process all the desired beneficial polymer properties should not be obtained at the expense of the overall catalyst productivity.